1. Field of the Invention
The present invention relates to density control.
2. Related Background Art
FIG. 9 is a sectional view showing the structure of a multicolor image forming apparatus. This prior art will be described below with reference to FIG. 9.
Latent images of different colors formed on an image carrier 100 by an internal semiconductor laser 204 of an optical unit 101 are developed into visual images by color toners of Y (yellow), M (magenta), C (cyan), and K (black) supplied from color developing devices Dy, Dm, and Dc, and Dk. These developed images are transferred onto the outer surface of a transfer belt 102 a plurality of times of rotation to form a multicolor image. That is, a high voltage is applied to the transfer belt 102 to transfer the toners onto the transfer belt 102.
A recording sheet 105 supplied from a paper supply unit 103 or a paper supply tray 104 is conveyed through a paper conveyance path, and the multicolor image is again transferred from the transfer belt 102 onto this recording sheet 105. After that, the recording sheet 105 is conveyed by conveyor rollers 106, fixed by a fixing unit 107, and delivered to a paper delivery tray 108 or a paper delivery unit 109.
Each color developing device has rotary support shafts on its two ends and is held in a developing device mechanism 110 so as to be rotatable on these rotary support shafts. The developing device mechanism 110 performs rotation control to select one of these color developing devices.
A cleaning unit 111 removes toner from the surface of the transfer belt 102. A waste toner unit 112 contains waste toner from the image carrier 100. A density sensor 113 senses the density of a toner image.
FIG. 10 is a block diagram showing an image data processing system of the multicolor image forming apparatus shown in FIG. 10.
A xcex3-conversion table 201 performs xcex3 conversion for image data VIN[7..0]. A dither conversion table 202 performs dither processing for this image data VIN[7..0]. A PWM circuit 208 modulates the pulse width of the image data VIN[7..0], thereby turning on and off the semiconductor laser 204 and printing the data.
The dither conversion table 202 expresses a gradation image by using a density matrix which is a set of a plurality of multilevel image dots. This dither conversion table 202 is composed of, e.g., an SRAM which converts the image data VIN[7..0] on the basis of dither matrix position information from a sub-scanning counter 205 and a main-scanning counter 206.
FIG. 11 is a view for explaining the way the dither conversion table 202 performs dither conversion. That is, FIG. 11 shows the relationship between the image data VIN[7..0], a conversion output VB[7..0] from the dither conversion table 202, and a print example obtained by pulse width modulation by the PWM circuit 203.
Even when a plurality of pixels collectively express the density, the relationship between the image data and the actual printing density is nonlinear, as shown in FIG. 12A. Hence, as shown in FIG. 12B, a halftone correction curve is derived by calculations to linearly correct the characteristic curve. This is the xcex3 conversion by the xcex3-conversion table 201. As an example, the xcex3-conversion table 201 is so set as to perform data conversion as shown in FIG. 13. This data conversion improves the linearity of the gradation of an image.
In the above prior art, to perform correction as shown in FIG. 12B, the xcex3-conversion table 201 is set as shown in FIG. 13 to perform data conversion, thereby performing halftone density correction.
Referring to FIG. 13, however, when VIN[7..0] is 80h to 84h, the conversion output VB[7..0] has the same value 72h. As is evident from this fact, data conversion (8 bits/8 bits conversion) is done by the image data bit width, so bit missing takes place. This reduces the number of gradation and forms a pseudo contour in a printed image.
It is, therefore, an object of the present invention to be able to obtain a high-quality halftone image by preventing bit missing when xcex3 conversion for dither matrix conversion is performed.
To achieve the above object, there is provided an image processing apparatus comprising a gradation converting unit for executing a gradation conversion process for input image data, and a dither converting unit for converting the gradation conversion processed image data into a plurality of image data by using a dither matrix, wherein the gradation conversion processed image data has the number of conversion errors due to the gradation conversion smaller than the number of gradation expressible by the dither matrix.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.